Shutter

ABSTRACT

A shutter for a light source is described, the shutter being switchable between a substantially opaque condition and a substantially transparent condition. In one embodiment the shutter comprises a layer of a liquid cyrstal material located between polarised elements, a control system being provided to control the orientation of the crystals of the layer. A shuttered light source incorporating the shutter is also discribed.

This arrangement relates to a shutter, and in particular to a shuttersuitable for use in preventing or reducing the emission of light from ahousing containing a light source.

Ultraviolet light is commonly used to assist in the curing of inks,adhesives, paints, varnishes or the like. In such applications, one ormore UV light sources are positioned adjacent the medium to which thematerial to be cured has been applied. The light sources used in theseapplications typically comprise mercury arc lamps. Such lamps require awarm-up or burn in period after being switched on before the desiredoutput frequency or frequencies are emitted thereby. Further, after use,a significant cool-down period must be provided before the lamp can bere-used or re-struck.

In order to control exposure times, to avoid damage to machinery, thematerial being cured or the medium to which material to be cured hasbeen applied, and to reduce or avoid the risk of exposure to humans, itis desirable to be able to stop the emission of radiation. However, asmentioned above, simply turning off the light source may be undesirable.In order to allow control over radiation without requiring the lightsource to be switched off, it is known to provide mechanical shutters.These shutters typically take the form of an opaque barrier materialsheet that can be moved between a stowed position and a position betweenthe light source and the medium by an appropriate actuator mechanism. Itwill be appreciated that when located between the light source and themedium, irradiation of the medium by the light source is prevented. Analternative arrangement includes using an actuator mechanism to move thelight source to a position adjacent an opaque barrier material, again toprevent irradiation of the medium by the light source.

The high heat levels generated during operation can result in failure ofthe actuator mechanism, and this is clearly undesirable.

Rather than provide a moveable shutter or fixed shutter position towhich a light source can be moved, it is known to provide UV lightsources designed to permit more rapid switching than can be achievedwith typical mercury arc lamps. However, these types of lamp often donot have the same spectral output as a conventional mercury arc lamp,and this may impair their performance in curing materials.

It is an object to provide a shutter whereby the disadvantages describedabove may be reduced or overcome.

According to the present invention there is provided a shuttercomprising a body switchable between a substantially opaque conditionand a substantially transparent condition, relative to a predeterminedfrequency of radiation.

Such a shutter may be rigidly fixed relative to a light source orrelative to a medium such that no additional actuator mechanism isrequired to control the position of the shutter.

The shutter may comprise two plane polarised elements, one of which ismoveable relative to the other to switch the shutter between itssubstantially opaque and substantially transparent conditions.

Alternatively, the shutter may comprise a solid state shutter, forexample comprising a plane polarised element and a medium, thepolarisation of which can be switched to control whether the shutter issubstantially opaque or substantially transparent. The medium maycomprise, for example, a layer of a liquid crystal material, in whichcase a control system for controlling the current or voltage applied tothe liquid crystal material and thereby control the orientation of thecrystals is provided. A second plane polarised element may be provided,the controllable medium being located between the plane polarisedelements

The control system is preferably arranged to allow parts of the shutterto be controlled independently of other parts thereof, such that theshutter may, at any given time, include one or more areas which aresubstantially opaque and one or more areas which are substantiallytransparent.

It will be appreciated that such an arrangement may be controlled so asto serve as a shutter and also to act, in effect, as a graduated filterallowing a larger intensity of radiation to be incident upon one part ofa target area as compared to that incident upon another part thereof.

In use, the heat generated by the UV source may be such that it isdesirable to provide a cooling system for the shutter. By way ofexample, cooling of the shutter may be achieved by causing a flow of airor water to pass over or adjacent the shutter.

The invention further relates to a shuttered light source comprising alight emitting device arranged to irradiate a target area, and a shutterof the type defined hereinbefore located between the light emittingdevice and the target area. The light emitting device is convenientlyarranged to emit ultraviolet light. The light emitting device may belocated within a housing having an opening formed therein, the shutterbeing located across or adjacent the opening to control the emission oflight through the opening. The housing may include or be defined, inpart, by a reflector arranged to reflect light towards the opening.

According to another aspect of the invention there is provided ashuttered light source having a filter through which at least aproportion of the radiation emitted by the light source passes, thefilter being graduated such that the intensity of radiation incidentupon one part of a target area differs from that at another part of thetarget area.

The filter may comprise a shutter of the type described hereinbefore.Alternatively, the filter may comprise a transparent substrate part orparts of which has or have been treated, for example by printing oretching, to render it or them less transmissive or opaque.

Where the shutter allows individual control over the pixels thereof,then the shutter may be controlled using software adapted to use digitalraster image or RIP raster image data, or similar image creation orprocessing techniques in controlling which of the pixels should beopaque and which should be transparent at any given time to allowcontrol over the UV curing dose applied to the image.

The invention will further be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a shuttered light source;

FIG. 2 is a diagrammatic sectional view illustrating a shutter inaccordance with one embodiment of the invention;

FIG. 3 is a diagrammatic sectional view of an alternative shutter; and

FIG. 4 is a diagrammatic view illustrating one mode of operation of theshuttered light source of FIGS. 1 and 2.

The arrangement illustrated in FIG. 1 comprises a housing 10 withinwhich is located a mercury arc lamp 12. A suitable control arrangement(not shown) for controlling and energising the mercury arc lamp 12 isprovided. A reflector 14 is provided within the housing 10 to direct thelight emitted by the mercury arc lamp 12 towards an opening 16 providedin the housing 10, and through the opening towards a target area. Ashutter arrangement 18 is located across the opening 16 to control theemission of radiation from the housing 10 towards a medium 20 located atthe target area. In the arrangement illustrated, the medium 20 is ofrelatively large dimensions compared to those of the housing 10 and as aresult, in order to ensure that the full area of the medium 20 can beirradiated, either the housing 10 is arranged to be moveable over themedium 20, and/or the housing 10 is fixed and a suitable drivearrangement is provided to move the medium 20. It will be appreciatedthat, depending upon the application in which the invention is to beused, the housing 10 could be associated with, for example, the printhead of an inkjet printer, the print head and the housing 10 beingtranslatable across the medium 20, the medium 20 being moveable, insteps, past the print head and housing 10 thereby allowing the fullsurface area of the medium 20 be irradiated over a period of time.

The shutter arrangement 18 comprises first and second transparentmembers 22, 24 each of which is plane polarised. Although the members22, 24 could take a range of forms, they conveniently take the form ofsheets of glass or quartz. Located between the sheets 22, 24 is a layer26 of a liquid crystal material. Electrodes 28 are provided around thelayer 26, energisation of the electrodes 28 being controlled by asuitable control arrangement 30. In use, the control unit 30 controlsthe energisation of the electrodes 28 to control the orientation of thecrystals of the layer 26 of liquid crystal material. Depending upon thetype of liquid crystal material used, the sheets 22, 24 may be arrangedsuch that the axes of polarisation thereof are parallel to one another,and such that when the electrodes 28 are de-energised light is able tobe transmitted through the shutter arrangement 18, energisation of theelectrodes 28 under the control of the control arrangement 30 causingthe crystals of the liquid crystal material to align in such a directionthat light transmission through the shutter arrangement 18 is no longerpermitted. It will be appreciated, however, that other arrangements arealso possible.

In use of the shuttered light source illustrated in FIG. 1 using theshutter arrangement 18 shown in FIG. 2, when it is desired to irradiatethe medium 20, the control arrangement 30 is operated to control theshutter arrangement 18 such that the shutter arrangement issubstantially transparent at least to the frequencies of ultravioletlight emitted by the light source 12. At times when the irradiation isno longer required, the control unit 30 is operated to render theshutter arrangement 18 substantially opaque at least to the ultravioletfrequencies of interest, thereby preventing irradiation of the medium20.

Although in a relatively simple arrangement the control arrangement 30may be arranged to control the shutter arrangement 18 such that the fillarea of the shutter arrangement is either opaque or transparent at anygiven time, more complex arrangements are possible in which the controlarrangement 30 controls the operation of the shutter arrangement 18 insuch a manner that only selected parts of the shutter arrangement 18 arerendered transparent or substantially transparent at any given time. Byway of example, where the light source is larger than as hereinbeforedescribed, and extends over the full width of the medium and protrudesbeyond the edges of the medium, then it may be desirable to ensure thatthe parts of the shutter arrangement 18 which extend beyond the edges ofthe medium remain opaque whilst the parts of the shutter arrangementlocated directly over the medium 20 can become transparent. Such anarrangement may be advantageous in that the exposure to radiation ofanything other than the medium can be avoided. Further, even where thelight source 10 extends only over the medium 20, rather than extendingbeyond the edges of the medium 20, there may situations where the areaof the medium 20 to be irradiated is less than the full area thereof,for example if the medium has margins to which ink or another curablematerial has not been applied, in which case certain parts of the edgeof the medium 20 may be shielded by ensuring that the associated partsof the shutter arrangement 18 remain substantially opaque. There mayfurther be situations where it is desired to shield other parts of themedium 20 from radiation and it will be appreciated that by appropriateoperation of the control arrangement 30, the shutter arrangement may becontrolled in such a manner that the relevant part or parts of theshutter arrangement remain opaque, in use. For example, when used in aprinting application, a dark part of an image may be formed be layingdown black ink to a depth of up to 30 μm, lighter parts of the imagehaving a relatively low density of ink dots, or even no ink at all. Inorder to avoid damage to the medium in areas in which little or no inkhas been delivered, whilst allowing curing of the darker parts of theimage, it may be desirable to use the technique mentioned hereinbefore.

Where the shutter arrangement 18 does not remain fixed relative to themedium 20, in use, then it will be appreciated that the controlarrangement 30 may be required to modify those areas of the shutterarrangement 18 which are opaque and those areas that are transparent asthe shutter arrangement is moving relative to the medium 20.

Depending upon the manner in which the shutter arrangement 18 iscontrolled, it may be possible to provide a sufficient number ofindependently controllable “pixels” in the shutter arrangement that, byappropriate control of the shutter arrangement and the pattern of thepixels rendered substantially opaque at any given time, the power outputof the light source can be controlled. The power output variationachieved in this manner need not be distributed evenly over the medium,but rather, if desired, some parts of the medium may receive a differentstrength “dose” to other parts thereof.

One area in which it is known to use ultraviolet radiation is in thecuring of ink delivered by an inkjet printer, for example of the drop ondemand (DOD) traversing type. It is common, in such printers, for themedium being printed to be moved in steps smaller than the width of thearea typically irradiated by a conventional UV light source, and for inkto be delivered over several passes or traverses of the print headrelative to the medium. As a result, ink delivered during one of theearlier passes may be irradiated several times before leaving the areabeing irradiated, whereas ink delivered during one of the later passesmay only be irradiated once. If the power output of the UV source ischosen to ensure that the ink delivered during one of the later passesis cured, then ink delivered during one of the earlier passes may beirradiated to such an extent that it degrades. Where the system of thepresent invention is used, then the shutter may be controlled in such amanner as to ensure that the ink delivered during the early passesinitially receives only a relatively low dosage of UV radiation byensuring that the part of the shutter located over this part of themedium contains only a relatively small area or proportion which istransparent. FIG. 4 illustrates a pixel configuration which may be usedto achieve the effect, but it will be appreciated that a number of otherconfigurations may be used within the scope of the invention.

In the arrangement shown in FIG. 4, the shutter has a plurality ofindividually controllable pixels switchable between a substantiallytransparent condition and a substantially opaque condition. In FIG. 4,pixels 40 are substantially transparent and pixels 42 are substantiallyopaque, this arrangement being achieved by appropriate control of theshutter. The effect of controlling the shutter to operate in this manneris to form, in effect, a graduated filter located between the lightsource and the target area such that whilst one part of the target areais subject to irradiation of relatively high intensity, other parts aresubject to lower intensities or irradiation whatsoever.

As mentioned hereinbefore, other patterns of pixels may be used toachieve this effect.

Rather than use the shutter to serve as a filter, it is possible to usea separate filter component. This could, of course, take the form of asecond component of construction similar to that of the shutter andcontrolled appropriately. Alternatively, a suitably graduated filtercomponent may be located between the light source and the target area.By way of example, the filter may comprise a glass or quartz substrateto which ink has been applied or another treatment used to vary thetransmissiveness of the substrate over the area thereof in a desiredpattern.

FIG. 3 illustrates an alternative to the shutter arrangement illustratedin FIG. 2. In the shutter arrangement shown in FIG. 3, two sheets 32, 34of plane polarised material, for example sheets of plane polarised glassor quartz, are located adjacent one another. An actuator arrangement 36is provided to control the angular position of one of the sheetsrelative to the other to control the orientation of the axes ofpolarisation of the two sheets. It will be appreciated that when theactuator arrangement 36 holds the sheets 32, 34 such that their axes ofpolarisation are perpendicular to one another, then the shutterarrangement is of substantially opaque form, movement of the sheetsrelative to one another such that their axes of polarisation liesubstantially parallel to one another resulting in the shutterarrangement becoming substantially transparent.

In the arrangements illustrated hereinbefore, it will be appreciatedthat as the shutter arrangement 18 is located permanently adjacent thelight source 12, the shutter arrangements 18 will be subject to highlevels of heat. In order to minimise the risk of damage to the shutterarrangements 18 due to the application of such high heat levels, ifdesired the housing 10 may have associated therewith a coolingarrangement arranged to force air or water over or past the shutterarrangements 18 to dissipate the heat absorbed thereby, in use.

Although in the description hereinbefore the shutter arrangement isdescribed as being located across an opening formed in a housing, itwill be appreciated that this need not be the case, the importantrequirement being that the shutter is so located relative to the lightsource and the medium to be irradiated that the shutter controls theirradiation of the medium without requiring switching of the lightsource between its on and off conditions.

Any of the shutters or shuttered light sources described hereinbeforemay be controlled by any suitable technique to ensure that irradiationof the target area occurs only at the desired time and, whereappropriate, to control the parts of the target area which areirradiated or to control the intensity of irradiation of various partsof the target area. The shutters or shuttered light sources may becontrolled, for example, by computer using software to relate a digitalraster image, or the output of RIP raster image processing or othersimilar techniques for creating or processing digital images to thetiming, power or polarisation of the individual pixels of the shutter toallow variation of the UV curing dose over parts of the image as theimage is moved relative to the shutter or shuttered light source, inuse.

It will be appreciated that the invention is not restricted to thespecific arrangements described hereinbefore, and that a number ofmodifications may be made within the scope of the invention.

1. A shutter comprising a body switchable between a substantially opaquecondition and a substantially transparent condition, relative to apredetermined frequency of radiation.
 2. A shutter according to claim 1,wherein the body comprises two plane polarised elements, one of which ismoveable relative to the other to switch the shutter between itssubstantially opaque and substantially transparent conditions.
 3. Ashutter according to claim 1, wherein the shutter is a solid stateshutter.
 4. A shutter according to claim 3, the solid state shuttercomprising a plane polarised element and a medium, the polarisation ofwhich can be switched to control whether the shutter is substantiallyopaque or substantially transparent.
 5. A shutter according to claim 4,wherein the medium comprises a layer of a liquid crystal material, acontrol system being provided for controlling the current or voltageapplied to the liquid crystal material to thereby control theorientation of the crystals.
 6. A shutter according to claim 5, furthercomprising a second plane polarised element, the controllable mediumbeing located between the plane polarised elements.
 7. A shutteraccording to claim 5, wherein the control system is arranged to allowparts of the shutter to be controlled independently of other partsthereof, such that the shutter may, at any given time, include one ormore areas which are substantially opaque and one or more areas whichare substantially transparent.
 8. A shutter according to claim 1,further comprising a cooling system for the shutter.
 9. A shutteredlight source comprising a light emitting device arranged to irradiate atarget area, and a shutter as claimed in claim 1 located between thelight emitting device and the target area.
 10. A light source as claimedin claim 9, wherein the light emitting device is arranged to emitultraviolet light.
 11. A light source as claimed in claim 9, comprisinga housing having an opening formed therein, the light emitting devicebeing located within the housing, the shutter being located across oradjacent the opening.
 12. A shuttered light source having a filterthrough which at least a proportion of the radiation emitting by thelight source passes, the filter being graduated such that the intensityof radiation incident upon one part of a target area differs from atanother part of the target area.
 13. A light source as claimed in claim12, wherein the filter comprises a shutter according to claim
 1. 14. Alight source as claimed in claim 12, wherein the filter comprises atransparent substrate part or parts of which has or have been treated,for example by printing or etching, to render it or them lesstransmissive or opaque.
 15. (canceled)
 16. (canceled)